1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of identifying a visited public land mobile network and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of the universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmissions/reception, uplink (UL) multiple-input multiple-output (UL-MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Device-to-device (D2D) communication may be realized, after an initialization (e.g., connection establishment and/or peer discovery) is performed (e.g., assisted by an eNB). Then, two UEs may communicate (e.g., transmitting and/or receiving packets) with each other directly according to the D2D communication, and the eNB does not need to forward the packets transmitted between the communication devices. According to the D2D communication, the UEs may communicate with each other via UL resources (e.g., UL subframes configured by the eNB). In general, the D2D communication may also be seen as a D2D service (i.e., proximity service (ProSe)). In addition, a D2D subframe, D2D transmission, D2D communication and D2D discovery can be termed as a sidelink subframe, sidelink transmission, sidelink communication and sidelink discovery, respectively.
An announcing UE can announce information which can be used by UEs in proximity if the UEs have permission for proximity discovery. A monitoring UE can monitor information of interest in proximity transmitted by announcing UEs. For the announcing UE, a ProSe Application Code can be received from a HPLMN ProSe function by performing an announce request procedure. The ProSe Application Code is included in a message which can be transmitted over a radio interface by a UE engaged in a ProSe direct discovery procedure to monitoring UEs. For the monitoring UE, Discovery Filter(s) for monitoring the ProSe Application Code(s) over the radio interface can be received from the HPLMN ProSe function when performing a monitor request procedure.
In prior art, when an announcing UE is registered in a visited public land mobile network (VPLMN), a ProSe function in a home PLMN (HPLMN) of a monitoring UE may provide information of a ProSe Application ID in a match report procedure to a ProSe function in the VPLMN of the announcing UE. If the announcing UE is roaming, the ProSe function in the HPLMN of the monitoring UE learns a VPLMN ID of a serving PLMN for the announcing UE in a monitoring request procedure.
However, the announcing UE may change the serving PLMN, before the monitoring UE detects a ProSe Application Code and starts the match report procedure. The VPLMN ID of the announcing UE stored in the ProSe function in the HPLMN of the monitoring UE may be out of date. The ProSe function in the HPLMN of the monitoring UE may send a notification message to a wrong ProSe function in a different VPLMN of the announcing UE.
Thus, how to handle the outdated VPLMN ID is an important problem to be solved.